Stents are a known class of surgical device for expanding or maintaining an open lumen or passageway through various body cavities, vessels or ducts. Locations in which stents are used include, but are not limited to, the urethra, the ureters, the common bile duct, the vagina, the cervix, the fallopian tubes, the sinus tract, the rectum, the bowel, the esophagus and the vascular tract.
While some stents are intended for long term use, there are numerous clinical applications in which stents are placed in the body only temporarily, for example, for temporary treatment of a medical problem, or to facilitate healing during a patient's recovery after a particular procedure. Laser ablation and cryoablation of the prostate are examples of such clinical applications. A short term (ten days to two weeks) indwelling stent ensures urethral and prostate patency after ablation.
It is highly desirable that any stents employed be removed from the body after such temporary uses. However, many stents possess open structures which permit the ingrowth or intrusion of tissue into them, or which permit the stents to embed themselves in the body tissues forming the cavities in which they are placed. Such ingrowth, intrusion or embedding makes it difficult to remove the stents without further trauma to the surgical site, causing discomfort to the patient and possibly delaying the patient's recovery from the particular procedure that has been performed.
For example, U.S. Pat. No. 4,503,569 (C. T. Dotter, Mar. 12, 1985) discloses a transluminally placed expandable graft prosthesis which includes a helically wound coil having a generally tubular shape. A version of a similar prosthesis is sold by Rabkin Corporation. The coil is made of a shape memory nitinol wire. The coil is placed in a blood vessel and is heated to its transition temperature, so as to expand and thereby keep the blood vessel open. However, the open configuration of the expanded coil permits prompt fibroblastic envelopment of the coil, firmly anchoring the coil within the blood vessel lumen. As a result, the coil cannot be removed from the body without surgical cutting.
Another type of helical vascular stent is disclosed in U.S. Pat. No. 4,820,298 (E. G. Leveen et al., Apr. 11, 1989) and includes a helix composed of a medical grade thermoplastic with strands of material extending between the spaced apart coils of the helix. The strands promote cellular ingrowth between the coils, so that removal of the stent similarly requires surgical cutting.
Yet another type of device is shown in U.S. Pat. No. 5,098,374 (E. Othel-Jacobsen et al., Mar. 24, 1992). A version of a similar stent is sold under the registered trademark "PROSTAKATH" by Endovision Pry. Ltd., Victoria, Australia. The device is a partial catheter formed of a wire coil having two pluralities of coil turns separated by a straight or rod-like segment. The catheter is asserted to be removable from the vessel or duct in which it is positioned by manipulation with forceps. However, because the spaces between the coil turns of the catheter permit tissue ingrowth and crystalline formation on and between the coil turns, the catheter becomes enmeshed with the vessel wall and is difficult to remove without trauma to the vessel wall, and bleeding and discomfort to the patient.
U.S. Pat. No. 4,580,568 (C. Gianturco, Apr. 8, 1986), No. 4,739,762 (J. C. Palmaz, Apr. 26, 1988) and No. 4,907,336 (C. Gianturco, Mar. 13, 1990) all disclose expanding wire stents of cylindrical shape, which are compressed by a removable sheath to facilitate their introduction into the lumen of a blood vessel or other body portion. (The first of these is sold under the name "Z-stent," a tradename of Cook Incorporated.) The sheath is withdrawn after the stent is positioned, and the stent self-expands ('568) or is expanded by a balloon catheter ('762 and '336) so as to keep the lumen open. Unfortunately, it has been found during the use of self-expanding stents of this and other types that the stents may continue to expand beyond the normal diameter of the passageway in which they are positioned. Thus, in some cases the stents become deeply embedded in the walls of the passageway. While this problem has been partially overcome by the use of flexible but inelastic rings to limit expansion of the stents, tissue ingrowth during their use remains a problem.
Tissue ingrowth can be obviated by the use of a variable diameter catheter, such as shown in U.S. Pat. No. 4,601,713 (C. R. Fuqua, Jul. 22, 1986), No. 4,710,181 (C. R. Fuqua, Dec. 1, 1987) and No. 4,738,666 (C. R. Fuqua, Apr. 19, 1988). Catheters of this type include a longitudinally extending folded wall, and during introduction into a body orifice the wall is held in place by a removable styler or sheath, whose removal allows expansion of the diameter of the catheter. Unfortunately, precisely because that wall is foldable, such catheters do not provide structural support for maintaining an open lumen in the body orifice. Moreover, such catheters lack any means to reduce the catheter diameter during removal of the catheter from the body orifice.